10 REM Anti VDU 21
   15 REM (c) 1989 Ben Dooks
   20 oldvec=!&20E AND &FFFF
   30 FOR I%=0 TO 3 STEP 3
   40 P%=&900
   50 [OPT I% 
   60 .newentry
   70 CMP #21
   80 BEQ bad
   90 CMP #156
  100 BEQ bad
  110 JMP ok
  120 .bad
  130 LDA #7
  140 .ok
  150 JMP oldvec
  160 ]NEXT
  170 !&20E=!&20E AND &FFFF0000 OR newentry